Vacuum heat treating furnaces which employ electrical resistance heating elements are well known. A typical vacuum furnace has a furnace wall and a hot zone chamber of a circular cross-section which houses a series of banks of axial-spaced electrical resistance heating elements suspended from an inner wall of the hot zone chamber by a series of support rods. A heating element is generally made from graphite or molybdenum or a metal alloy, and generates radiant heat in response to electrical current passing therethrough. Popular designs are presented in U.S. Pat. No. 4,559,631 and in U.S. Pat. No. 4,259,538 (hereafter “the 538 patent”). The heat treating industry has benefited from reduced cost resulting from increased efficiencies in furnace performance resulting from inventions such as those described in: U.S. Pat. No. 6,021,155, “Heat Treating Furnace Having Improved Hot Zone” (hereafter “the 155 patent”), U.S. Pat. No. 6,023,487, “Process for Repairing Heat Treating Furnaces and Heating Elements Therefor” (hereafter “the 487 patent”), and U.S. Pat. No. 6,111,908, “High Temperature Vacuum Heater Supporting Mechanism with Cup Shaped Shield” (hereafter “the 908 patent”). Reduced cost has been a factor in creating larger demand for heat treating services. The services for “heat treatment” and “heat treating” as used in herein, unless otherwise specifically stated, refers to heat treatment under high vacuum, which includes both heating in the presence of selected gaseous environments, as well as high vacuum heating for brazing runs. Even though demand for heat treatment is high, competitive forces still require ever-increasing efficiencies. Larger furnaces have helped in response to that requirement. However, traditional mechanisms for loading target material pieces onto an internal furnace hearth become cumbersome, timely and/or potentially dangerous when used for loads having very heavy pieces. (“Target material” as referred to herein is the metal, ceramic or other material that is to be heat treated.) For example, even with specially designed fork lifts, loading the furnace is impractical with very heavy objects, e.g., target material pieces weighing 15,000 pounds. Currently employed lifts also create hazards to furnace elements (and other protrusions from the furnace inner wall) in loading and unloading large or heavy target materials that leave less room for vertical and/or horizontal tolerance. In addition to the above-described demand for treating larger target material pieces, I have found that there is a latent increased demand for treating larger loads (total size and/or weight). Existing furnaces rarely have a hot zone longer than 12 feet. Hence, it would be desirable to have a system that can safely load large or heavy target material into high temperature vacuum furnaces. It would also be desirable to provide a system for loading such material without major risk to furnace internal components. Because planarity of the furnace hearth is very important in many heat treating applications, it would also be desirable to provide a system that is robust and structured to accommodate precise hearth planarity.
One major limitation in designing a system to meet the above requirements has been difficulties associated with the apparent requirement of including any moving parts in the furnace hot zone. However, the extreme environments to which all parts are subjected in the hot zone (in access of 2000 degrees Fahrenheit, and very deep vacuum, e.g., up to 10−5 Torr) would cause lubricant evaporation and galling. Using “sealed” bearings cause their own problems (the bearing chamber may explode) under such drastic conditions.
The present invention describes a system for loading and unloading high temperature furnaces which is safe, productive and non destructive. The system also can handle heavy loads (for example, a total load of as much as 50,000 pounds). The new system can also load bulky materials while moving them in close proximity to internal protrusions, e.g., heating elements, (for example, a few inches) without concern for damage to the furnace. In another embodiment this invention provides the opportunity to minimize intrusion on valuable furnace time by minimizing time the furnace has to be open for the loading and unloading process. In yet another embodiment this invention provides a large robust hearth with an under-girding structure that supports high hearth planarity even when cycled through very high temperatures required for heat treating.